1. Field of the Invention
This invention relates to systems for measuring liquid velocity and in particular for measuring blood velocity on backscatter Doppler principles.
2. Description of the Prior Art
It is known to use ultrasonic devices for measurement of the speed of a liquid flow in general. These devices, most of which use two transducers, or sometimes a single transducer, use the Doppler effect. Devices are also known which apply the same principle to blood flow rate measurement. The known devices are speed meters; see particularly the articles by D. L. Franklin and collaborators in "The American Journal of Medical Electronics", 1st term 1966, pages 24-28, and "IRE Transactions on Bio-medical Electronics", January 1962, pages 44-49. As shown in FIG. 1, such ultrasonic devices typically included an ultrasonic transducer illustratively in the form of a crystal 14' that is energized by a generator 12' to emit ultrasonic waves into a conduit 24', whereby it is reflected by the liquid directed therethrough to be sensed by a detector, typically in the form of a crystal 18', the output of which is connected to a receiver 20'. The receiver 20', as will be explained below, detects the received Doppler signal which contains the velocity information. In the particular illustrative context of this invention, such principles are used to measure the velocity of blood as would be directed through a conduit external of the patient's body. Illustratively, it is contemplated that a heart assist machine would be used to aid the patient's heart or a dialysis machine be used to clean a patient's kidneys, and that the blood flow through such machines could be measured in accordance with the teachings of this invention.
The practical application of the Doppler backscatter principle consists of transmitting an ultrasonic beam into the medium whose velocity one wishes to measure and to compare the original frequency with the received shifted frequency. To retrieve the velocity information, a comparison of the scattered signal frequency with the original frequency is made by the receiver 20'. The difference in frequency is related to the flow velocity of the medium. Since the medium is flowing generally in a conduit of known dimension, the velocity information can be translated into total flow rate.
There are two basic aspects to this phenomena; the particle size can be larger than the wavelength of the transmitting ultrasound or it can be smaller, therefore acting as a point scatterer. It is noted that the red cells of blood have a typical diameter of 8 .mu.m, thickness of 2 .mu.m, with the wavelength of the 3.13 MHz ultrasonic beam being approximately 480 .mu.m in blood. In the case of red cells, the cell is smaller than the wavelength of the beam and the cell is set into motion and becomes a secondary emitter acting as a point source.
The envelope of the received signal represents the heterodyne coupling of the transmitted carrier with the backscattered signal whose normal frequency has been shifted by the Doppler phenomena. This signal then is amplified, demodulated, audio amplified, processed and displayed as flow rate by the receiver 20', as shown in FIG. 1. The shift in frequency is due to the relative motion of the object with respect to the transmitter and receiver. The frequency shift due to motion of the particle with respect to the transmitter is: ##EQU1## where
f.sub.1 = Frequency of the forced particle oscillation
V.sub.O = Ultrasound velocity in medium
V = Particle velocity
.eta. = Angle between the ultrasound and the velocity vector
f.sub.c = Ultrasonic carrier frequency
The frequency shift due to motion of the particle with respect to the receiver is: ##EQU2## where
f.sub.1 = Frequency of the forced particle oscillation
f.sub.2 = New frequency as measured at the receiver V, V.sub.O as indicated above
Combining (1) and (2), the total Doppler shift may be expressed as: EQU f=(F.sub.c -F.sub.1)+(f.sub.1 -f.sub.2)=f.sub.c -f.sub.2 ( 3) ##EQU3## The formula can be expanded in a series and only the most important term taken when V.sub.O (1500 m/sec) &lt;&lt;V&gt;&gt;V (1.5 m/sec at 10 L/minute), to provide the expression: ##EQU4## This is the general formula used in the backscatter Doppler flowmeter design. This signal is difficult to detect since the signal amplitude at this shifted frequency is small and becomes swamped by the direct coupled ultrasonic carrier frequency. Fortunately, the direct radiated ultrasonic wave received is added with the backscatter signal in the crystal 18' producing an amplitude modulated signal that retains all the basic information as indicated by formula (5). The receiving crystal 18' converts the ultrasonic energy back into an electrical signal. The amplitude modulated signal, at microvolt levels, is RF amplified, detected, audio amplified, processed and displayed as flow information.
In the above-referenced application, entitled "Liquid Velocity Measuring System", there is described a liquid and in particular a blood velocity measuring system comprising a first, transmitting transducer in the form of a crystal and energized by a continuous wave generator to generate an ultrasonic signal of a frequency of 3.13 MHz. The ultrasonic signal is directed into a conduit through which blood is passed, the ultrasonic wave being reflected by the blood cells of the blood directed through the conduit, to be detected by a second or receiver transducer. The reflected ultrasonic wave is frequency-shifted dependent upon the velocity of the blood directed through the conduit. Subsequently, the signal derived from the second ultrasonic transducer is amplified and demodulated before being applied to a frequency-to-voltage converter, the amplitude of whose output provides an analog manifestation of the velocity or rate flow of the blood through the conduit.
It is desired to provide a liquid or blood velocity measuring apparatus that is adapted to measure velocity or rate flow of a liquid through any of a plurality of conduits. In this regard, it is understood that as the diameter or cross-section of a conduit is decreased, that, for a given flow of liquid, its velocity increases. Conversely, as the cross-sectional area of a conduit is increased, the velocity of the liquid is decreased, where there is a constant flow or quantity of liquid directed through the conduit.
Further, it is desired to provide a portable liquid velocity system that may be energized by a self-contained power source such as a battery. When using batteries, it is desired to minimize the energy drained therefrom in order to extend their life. In considering the use of batteries to energize a liquid velocity measuring system of the type described above, it is contemplated that the crystal-type transmitter and in particular the generator for energizing such a transmitter at a high frequency, imposes a large current drain on a battery. Thus, it is desired to only intermittently operate such systems in order to measure liquid velocity upon command of the system's operator. In one particular contemplated environment, a portable blood velocity measuring system could be used in a hospital, whereby an attendant would carry the blood velocity measuring system from patient to patient. In particular, the contemplated apparatus would be most suitable for measuring blood flow through a conduit of a dialysis machine or heart assist machine, and could be readily transported from one machine to the next.
In U.S. Pat. No. 3,741,014 of Tamura, there is described an ultrasonic current meter for measuring the flow rate of a fluid utilizing the Doppler phenomena. In particular it is contemplated that an oscillator or generator would energize an ultrasonic transmitter to direct an ultrasonic wave into a liquid, to be reflected by the liquid and received by a receiver transducer. The output of the receiver transducer is amplified and mixed with a signal as derived from the transducer oscillator or generator. In turn, the output of the mixer is applied subsequently to a detector and to a "signal pulse converter". The output of the converter comprises a train of signals proportional to the Doppler frequency and is multiplied by a scale factor and converted into a binary decimal code by the converter to provide a digital output signal that may be displayed by a digital indicator.
In U.S. Pat. No. 3,921,622 of Cole, there is described a system for transmitting a continuous ultrasonic signal across a conduit through which a fluid is directed to detect changes in the amplitude of the received signal, indicative of changes in its acoustic impedance and therefor the presence of bubbles within the liquid flow. If the impedance of the liquid increases above a predetermined level as determined by a trigger circuit, a clock circuit is connected to a counter, whereby the number of clock signals occurring after the detection of the bubbles is counted.